Water use efficiency in tropical plants based on a set of newly created leaf photosynthesis-related parameters

Water use efficiency (WUE) quantifies the amount of water expended per unit of dry leaf matter accumulated, reflecting the trade-offs between water consumption and carbon uptake. It is also a critical parameter for understanding plant responses to environmental changes. This study introduced an innovative set of WUE-related parameters, including maximum water use efficiency (WUE_max) and associated coefficients of water potential, loss, strategic usage, and total usage (WPC, WLC, WSC, and WTC, respectively) in providing a comprehensive evaluation of water use strategies in 48 common tropical plant species during the natural light fluctuations. These parameters exhibited significant differences among plant types, with sun-adapted and shade-tolerant plants (both C3) showing mean of WUE_max values of 3.81 ± 0.63 and 5.42 ± 1.61 μmol mmol^-1, respectively, whereas C4 plants demonstrated a greater WUE_max of 7.04 ± 1.77 μmol mmol^-1. Compared to C3 plants, particularly shade-tolerant types, C4 plants exhibited significantly higher WPC and WTC (p < 0.05). Furthermore, shade-tolerant plants displayed lower WLC and higher WSC than sun-adapted plants, suggestive of their specialized adaptations to variations in light intensity. The sensitivity of stomatal and mesophyll conductance (i.e., g_s and g_m) to incident light (I_inc) and/or intercellular CO₂ concentration (C_i) helped clarify the source of variation in WUE-related parameters. In sun-adapted plants, g_s was sensitive to changes in both I_inc and C_i. In terms of g_m, shade-tolerant plants exhibited the lowest overall sensitivity to I_inc. Increasing atmospheric CO₂ concentrations from 400 to 450 ppm caused WUE-related parameters to vary, with this response differing among plant types. These insights emphasize the significance of plant adaptation strategies in tropical rainforest ecosystems.